The present invention is directed, in general, to power electronics and, more specifically, to a local loop control system and a multiple output power converter employing the same.
Multiple output switching converters are often employed to provide multiple output voltage levels from a single transformer with the outputs consisting of a main output and at least one auxiliary output. Typically, the main output is designed to operate at a higher voltage (e.g., a higher DC voltage) than the auxiliary output. When changes in load conditions occur, multiple output converters endure output voltage variations that are detrimental to the overall operation of the converter. In switching converters, a controlling parameter, such as the duty cycle of the main power switch therein, may be regulated to either increase or decrease the voltages at the main and auxiliary outputs, concurrently. When only one output is regulated, however, all other outputs may be adversely affected with load variations. The problem is exacerbated when the load requirements at the outputs move in opposite directions, since each output demands a conflicting change in duty cycle of the main power switch to adjust the output voltage to the desired value.
Common techniques for improving output regulation of a multiple output converter include weighted sum averaging, series pass regulation and shunt regulation. Weighted sum averaging allows improvement of the output regulation of one output at the expense of the other. A series pass regulator may be used to regulate small amounts of current. As current requirements increase, the series pass regulator becomes more expensive and the power losses increase. Shunt regulators divert current through parallel impedances in order to vary the output voltage. The current is shunted to the ground or common side and power is dissipated. Each of these techniques offers trade-offs between the efficiency and regulation of the converter.
Another technique used in multiple output converters is called a current diverter circuit (see, for instance, U.S. Pat. No. 5,991,168 entitled xe2x80x9cTransient Response Network, Method of Diverting Energy in a Multiple Output Power Converter Employing the Same,xe2x80x9d by Farrington, et al., issued Nov. 23, 1999, which is incorporated by reference). The current diverter circuit is basically a shunt regulator for application to multiple output converters that do not have independent regulation of the separate outputs. The current diverter circuit intermittently or continuously bleeds current from the higher voltage output to the lower voltage output. This technique, although an improvement from an efficiency perspective over straight shunt or series regulators, results in a continuous or continuing power loss with a resulting degradation of the power unit efficiency. Additionally, there is a practical limit to the maximum power that can be bled from one output to the other.
Multiple output converters which employ independent precision regulators for each output are able to achieve accurate regulation of both outputs. Typically, the main output voltage is controlled by regulating the energy applied to the transformer using pulse width modulation control of the main power switch on the primary side of the transformer. The other or auxiliary output, on the other hand, is regulated independently on the secondary side of the transformer. The independent regulation may be a magnetic amplifier or an equivalent active circuit.
A particularly troubling problem exists in multiple output converters employing independent precision regulators when a significant portion of the load on the main output decreases. When the load is removed or greatly reduced, the regulating circuit for the main output drops to a minimum duty-cycle condition. Since the auxiliary output also receives its input from energy transferred across the transformer, the independent regulating means for the auxiliary output typically cannot compensate for the substantial decline of the regulating signal for the main output. Consequently, the auxiliary output drops out of regulation for a period of time causing an unacceptable decrease in the auxiliary output voltage. If the auxiliary output is supplying power to a computer circuit, for instance, the loss of regulation, even for a brief period of time, can have catastrophic consequences. These consequences often cannot be corrected even when the load at the main output returns to an acceptable level and regulation of the auxiliary output is restored.
As synchronous rectifiers are introduced into the design of multiple output converters to improve efficiency, there is an opportunity to use them in a dual role of improving efficiency and adjusting their forward drop to provide improved voltage regulation. Although this approach has been previously proposed [see U.S. Pat. No. 5,999,417, entitled xe2x80x9cHIGH EFFICIENCY POWER CONVERTERxe2x80x9d, to Martin F. Schlecht, issued Dec. 7, 1999, which is incorporated herein by reference], it typically generates a conflict between the primary voltage control loop and the local regulation loops as each attempts to adjust the output voltages. This condition introduces an opportunity for instability, which may create correspondingly difficult design compromises. A control process that would allow the principal power control and the separate output controls to effectively maintain the required outputs while minimizing conflict and without sacrificing overall efficiency is highly desirable.
Accordingly, what is needed in the art is a way to preserve the overall efficiency of a multiple output converter while effectively maintaining regulation of the output voltages.
To address the above-discussed deficiencies of the prior art, the present invention provides a local loop control system and method of operating the same for use with a power converter. The power converter includes a first output coupled to a first synchronous rectifier circuit, a second output coupled to a second synchronous rectifier circuit and a main controller configured to regulate a voltage proportional to a weighted sum of the first and second outputs. In one embodiment, the local loop control system includes a first output controller configured to regulate a voltage at the first output and a second output controller configured to regulate a voltage at the second output. One of the first and second output controllers is configured to reduce a drive signal to a corresponding one of the first and second synchronous rectifier circuits when a corresponding voltage at one of the first and second outputs exceeds a voltage proportional to the weighted sum.
In another aspect, the present invention provides a method of operating a local loop control system for use with a power converter. The power converter includes a first output coupled to a first synchronous rectifier circuit, a second output coupled to a second synchronous rectifier circuit and a main controller configured to regulate a voltage proportional to a weighted sum of the first and second outputs. The method of operating the local loop control system includes regulating a voltage at the first output with a first output controller and regulating a voltage at the second output with a second output controller. One of the first and second output controllers reduces a drive signal to a corresponding one of the first and second synchronous rectifier circuits when a corresponding voltage at one of the first and second outputs exceeds a voltage proportional to the weighted sum.
The present invention also provides, in another aspect, a power converter that includes a transformer having a primary If winding and first and second secondary windings. The power converter also includes a primary power stage, coupled to the primary winding, associated with a main controller that controls a primary power switch to regulate a voltage proportional to a weighted sum of first and second outputs of the power converter. The power converter further includes a first secondary stage, coupled to the first secondary winding, having a first synchronous rectifier circuit coupled to a first output controller that regulates a voltage at the first output. The power converter still further includes a second secondary stage, coupled to the second secondary winding, having a second synchronous rectifier circuit coupled to a second output controller that regulates a voltage at the second output. One of the first and second output controllers reduces a drive signal to a corresponding one of the first and second synchronous rectifier circuits when a corresponding voltage at one of the first and second outputs exceeds a voltage proportional to the weighted sum.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.